Datasheet

LT3791
19
3791fa
For loop stability a lowpass RC filter is needed. For
most applications, a 50Ω resistor and 470nF capacitor
is sufficient.
Table 3
R
IN
(mΩ) I
LIMIT
(A)
20 2.5
15 3.3
12 4.2
10 5.0
6 8.3
5 10.0
4 12.5
3 16.7
2 25
IVINMON
The IVINMON pin provides a linear indication of the current
flowing through the input. The equation for V
IVINMON
is
V
(IVINP-IVINN)
• 20. This pin is suitable for driving an ADC
input, however, the output impedance of this pin is 12.5kΩ
so care must be taken not to load this pin.
Programming Output Overvoltage Threshold for Open
LED Protection
For an LED driver application with small output capacitors,
the output voltage usually overshoots a lot during an open
LED event. Although the 1.2V (typical) FB regulation loop
tries to regulate the output, the loop is usually too slow to
prevent the output from overshooting. Once the FB voltage
hits its overvoltage threshold, 1.25V (typical), the LT3791
stops switching by turning TG1, TG2 off, and BG1, BG2
on. In this way, the minimum overshoot is guaranteed.
The output overvoltage threshold can be set by selecting
the values of R5 and R6 (see Figure 9) according to the
following equation:
V
OUT(OVP)
= 1.25 •
R5
+
R6
R6
Make sure the expected V
FB
during normal operation does
not exceed 1.1V:
V
LED
•
R6
R5+R6
≤ 1.1
These equations set the maximum LED string voltage with
open LED protection for the LT3791 to be 52V.
Dimming Control
There are two methods to control the current source for
dimming using the LT3791. One method uses the CTRL
pin to adjust the current regulated in the LEDs. A second
method uses the PWM pin to modulate the current source
between zero and full current to achieve a precisely pro-
grammed average current. To make PWM dimming more
accurate, the switch demand current is stored on the V
C
node during the quiescent phase when PWM is low. This
feature minimizes recovery time when the PWM signal goes
high. To further improve the recovery time a disconnect
switch may be used in the LED current path to prevent the
ISP node from discharging during the PWM signal low
phase. The minimum PWM on- or off-time is affected by
choice of operating frequency and external component
selection. The best overall combination of PWM and
analog dimming capabilities is available if the minimum
PWM pulse is at least six switching cycles and the PWM
pulse is synchronized to the SYNC signal.
applicaTions inForMaTion
Figure 9. Resistor Connection for Open LED Protection
LT3791
V
OUT
R5
R6
3791 F09
FB